1. Field of the Invention
The present invention is generally related to measuring transmitted or reflected optical null-lens wavefronts, and more particularly to measuring the transmitted wavefront of a refractive null-lens or a computer-generated hologram (CGH).
2. Background Description
A typical aspheric optical surface has manufacturing errors or figure errors, i.e., errors between the surface design figure (the intended surface shape) and the manufactured surface shape. Measuring those errors has been challenging. The more aspheric the optical surface, i.e., the more the surface deviates from a spherical shape, the more challenging the measurement. Null-lenses are used to measure the figure of an asphere and its optical flaws or defects with great precision during optical element fabrication. Each null lens must be custom designed for the aspheric optic being measured and is designed to largely cancel out (i.e., null) a reflected or transmitted wavefront. Generally, null-lens design complexity increases with the numerical aperture of the manufactured aspheric optic. Such null-lens designs can include two or more custom-made precision optical components that require precise alignment to one another. Null-lenses must be accurately aligned to both a reference source and the aspheric optic under test.
Null-optics (also called null correctors or null lenses) have typically been used for measuring low-spatial-frequency fabrication errors (also called figure errors) during aspheric optical fabrication and testing. Measuring light passes through both the null lens and the aspheric optic determines the optical wavefront of the combined optic system, i.e., the null-lens and aspheric optic. Understanding the performance of the aspheric optic alone, however, is the final goal of the measurement. Typically, the combined optic system wavefront is measured using one of three methods: image-based wavefront sensing or phase retrieval; interference/fringe-based wavefront sensing, including interferometry; and, sub-aperture imaging and centroiding, e.g., with Shack-Hartmann wavefront sensors. For an example of testing of an optical surface figure using image-based wavefront sensing in metrology, see, e.g., G. R. Brady, “Application of Phase Retrieval to the Measurement of Optical Surfaces and Wavefronts,” Ph.D. dissertation, University of Rochester, 2008; and see, G. R. Brady and J. R. Fienup, “Measurement Range of Phase Retrieval in Optical Surface and Wavefront Metrology,” Appl. Opt. 48, 442-449 (2009).
Typically the null-lens has comparable aberrations to the aspheric optic it is designed to measure. Those aberrations alter the light traveling through the component(s), i.e., referred to as the optical wavefront, phase, or optical path difference (OPD). Sources for unknown, or unintended, aberrations include, for example, component manufacturing defects, assembly/integration defects, or errors from misalignment to the reference source or the aspheric optic under test.
There are four categories of null-lens wavefront components: design or nominal wavefront for the individual lens or individual component lenses; wavefront errors from manufacturing imperfections in each individual component lens; wavefront errors from individual null-optic element misalignments; and, test-alignment wavefront errors from null-lens misalignment relative to the reference source or the aspheric optic under test. Of these four categories, previously test alignment wavefront errors have been unmeasurable because these errors arise from how the end user uses the null-lens, e.g., during aspheric optic fabrication and testing, typically to measure figure errors. The test alignment wavefront errors were unmeasurable by the null-lens manufacturer, because the errors are post-delivery in end user alignment.
Since null-optic test-alignment aberrations were unmeasurable, previously, during the testing they would have been misattributed to the aspheric optic being measured. Consequently, it is important to identify and verify null-optic aberrations to precisely measure aspheric optics.
Thus, there is a need for measuring null-optic aberrations, and more particularly, for measuring null-lens design, manufacturing imperfection, and alignment wavefronts.